1. Field of the Invention
The invention relates to methods and apparatus for detecting torsional vibration in a bottomhole assembly mounted on the drill string of a rotary drilling system for drilling in an earth formation. As is well known, a rotary drilling system is a system in which the bottomhole assembly, including the drill bit, is mounted on a drill string which extends downhole and is rotated from the surface.
2. Description of Related Art
The invention is particularly, but not exclusively, applicable to bottomhole assemblies including rotary drag-type drill bits of the kind comprising a bit body having a shank for connection to a drill collar on a drill string, a plurality of cutters mounted on the bit body, and means for supplying drilling fluid to the surface of the bit body to cool and clean the cutters and to carry cuttings to the surface. In one common form of bit some or all of the cutters are preform (PDC) cutters each comprising a tablet, usually circular or part-circular, made up of a superhard table of polycrystalline diamond, providing the front cutting face of the element, bonded to a substrate, which is usually of cemented tungsten carbide.
While such PDC bits have been very successful in drilling relatively soft formations, they have been less successful in drilling harder formations or soft formations which include harder occlusions or stringers. Although good rates of penetration are possible in harder formations, the PDC cutters may suffer accelerated wear and bit life can be too short to be commercially acceptable.
Studies have suggested that the rapid wear of PDC bits in harder formations can be due to damage of the cutters as a result of impact loads caused by torsional vibration of the bottomhole assembly.
Torsional vibration can have the effect that cutters on the drill bit may momentarily stop or be rotating backwards, i.e. in the reverse rotational direction to the normal forward direction of rotation of the drill bit during drilling. This is followed by a period of forward rotation of up to twice the RPM mean value. It is believed that it is this behaviour which may be causing excessive damage to the cutters of PDC bits when drilling harder formations where torsional vibration is more likely to occur. The effect of reverse rotation on a PDC cutter may be to impose unusual loads on the cutter which tend to cause spalling or delamination, i.e. separation of part or all of the polycrystalline diamond facing table from the tungsten carbide substrate.
If it is known that torsional vibration is occurring in the bottomhole assembly, it may be possible for the operator of the rotary drilling system, at the surface, to reduce or stop the vibration by modifying the drilling parameters, for example by changing the speed of rotation of the drill string (RPM) and/or the weight-on-bit (WOB). However, it has hitherto been difficult to detect at the surface torsional vibration which is occurring in the bottomhole assembly, since many different frequencies of vibration may be transmitted to the surface and the high frequency vibrations become very attenuated as they pass upwardly along the drill string so that the amplitudes are much reduced at the surface. Accordingly, it has not been reliably possible, hitherto, to detect the onset of torsional vibration of the bottomhole assembly (except very low frequency vibrations which are dependent on depth) by monitoring general torque levels at the surface. It is possible to monitor torsional vibration of the bottomhole assembly by sensors located downhole, in the assembly itself, and transmitting signals from the downhole sensors to the surface. While this may be done in test rigs, it is not generally a practical proposition in commercial drilling.
It would therefore be desirable to be able to monitor torque vibration in the drill string, at the surface, in such a manner that the presence of torsional vibration in the bottomhole assembly can be detected at the surface, and it is this problem which the present invention sets out to solve.
The present invention is based on the realization that the frequencies of torsional vibrations of a bottomhole assembly are associated with the natural resonance frequencies of the drill collars and other components of the bottomhole assembly, and particularly in the modes which involve integer wavelengths, e.g. one or two full wavelengths, of the bottomhole assembly only. The frequencies of these modes can be calculated from the geometry of the bottomhole assembly alone and do not depend on local drilling parameters. The present invention is therefore based on the concept of monitoring at the surface only those frequencies which are in the region of the natural frequencies of the bottomhole assembly.